Method for carburizing and carbonitriding steel by carbon oxide

ABSTRACT

The method for enriching steel with carbon by carbon oxide comprises a step involving heating to a temperature of enrichment in a furnace in a gaseous phase, then an enrichment step involving the introduction of a gas containing carbon oxide. According to the invention, atomic hydrogen is formed in said gaseous phase prior to the enrichment phase, said atomic hydrogen being of a sufficient amount to suppress interface resistance to the transfer of carbon and to avoid surface oxidation of said steel.

[0001] The invention relates to the field of metallurgy and it deals with a method of carburizing and carbonitriding steels with carbon monoxide, providing improved rate of carbon transfer into the steel and avoiding surface oxidation of the carburized and carbonitrided layers.

[0002] Carburizing of steels with a gas-solid reaction using carbon monoxide is an old procedure, the methods with which it is carried out are well known and it is actually the most frequently utilized procedure on an industrial scale in batch furnaces or continuous furnaces, operating at atmospheric pressure with gaseous atmospheres consisting mainly of carbon monoxide and hydrogen. The carburizing potential of these gaseous mixtures is defined by the carbon potential of the atmosphere, a magnitude which characterizes the thermodynamic equilibrium state between the atmosphere and the steel at the treatment temperature. This carbon potential, which is a fundamental characteristic that permits assurance of control of the carburizing capacity of the atmosphere, also permits the control of side reactions, which are frequently undesirable, such as surface oxidation of the carburized or carbonitrided layers. As a matter of fact, carbon transfer between the gaseous atmosphere and the steel is carried out based on decomposition of the carbon monoxide molecule at the gas-solid interface. The carbon liberated by this decomposition diffuses into the austenitic solid solution, there creating a carbon concentration gradient.

[0003] The oxygen liberated concomitantly remains first adsorbed on the steel surface, and, depending on the specific characteristics of the atmosphere and of the treatment reactor (furnace), it combines

[0004] either with the molecular hydrogen of the atmosphere with the formation of water molecules in the atmosphere,

[0005] or with carbon monoxide to form molecules of carbon dioxide,

[0006] or with other oxygen atoms to form oxygen molecules.

[0007] None of these reactions are instantaneous. As a result, the oxygen liberated by the decomposition of carbon monoxide is only partially eliminated from the surface and thus a layer of adsorbed oxygen remains on this surface, and this layer behaves as a resistance to the transfer of carbon between the atmosphere and the steel. When the steel contains alloying elements which have a strong affinity to oxygen, such as chromium, manganese and silicon, it leads to intergranular surface oxidation, which increases with decreasing carbon potential. In this case, the concentration of carbon on the surface is low and the partial pressure of oxygen in the atmosphere is high.

[0008] The objective of the invention is to reduce the adsorbed oxygen layer, first of all to obtain a maximum weight of transferred carbon by reduction of the resistance to transfer, and, secondly, to avoid any intergranular surface oxidation.

[0009] The considerations developed above show that carbon enrichment should lead to as high carbon potential as possible because the partial pressure of oxygen in the atmosphere is then the lowest and the concentration of carbon on the steel surface is the highest.

[0010] However, there is an initial phase of the treatment during which the carbon potential of the atmosphere is low. The surface concentration of carbon in the steel is limited at the beginning of the treatment to the carbon content of the chosen steel (0.10 to 0.3% of carbon). It is during this initial phase that the adsorbed oxygen layer is established and may lead to oxidation of the surface and to a practically irreversible resistance to transfer.

[0011] This phenomenon is characteristic for all carburizing processes with carbon monoxide, used traditionally.

[0012] Therefore, the objective of the invention is to make the initial transient phase as short as possible and to use a steel surface where the adsorbed oxygen is eliminated with a powerful reducing agent.

[0013] According to the invention, this objective is realized by the introduction of atomic hydrogen into the atmosphere at the beginning of the treatment, before the carbon potential of the atmosphere reaches a very high value.

[0014] According to a first embodiment, atomic hydrogen is obtained by the thermal decomposition of a gaseous molecule with the formation of atomic hydrogen, at least transitorily. This decomposition occurs at the chosen temperature for enrichment of carbon and under conditions that the other species resulting from the decomposition of the molecules do not react with the steel.

[0015] According to a second embodiment, the atomic hydrogen is obtained by the decomposition of gaseous mixtures containing molecular hydrogen, in an electric or electromagnetic field in the form of cold plasmas or hot plasmas obtained with the aid of various modes of excitation, such as arc, microwave, radiofrequency, for example, in discharge or post-discharge.

[0016] An example of the method according to the invention is described below for the case of carburizing with carbon monoxide at atmospheric pressure:

[0017] the pieces of steel are introduced into the furnace where they are heated to the temperature of carbon enrichment. This is at least equal to 920° C. The atmosphere of the furnace consists of nitrogen which is chemically inert toward the steel during this phase of heating,

[0018] starting at 920° C., a flow of ammonia corresponding to a renewal of atmosphere of about 3 times in the furnace is introduced over a period that can reach 30 minutes. The ammonia is introduced during which the temperature of the charge continues to increase until the effectively chosen carbon enrichment temperature is reached, which is between 930 and 980° C.,

[0019] when the temperature is reached, the carburizing mixture (carbon monoxide-hydrogen) is introduced at a flow rate which is sufficient to reach a concentration of carbon monoxide in the gaseous mixture of 24% in a few minutes (5 to 10 minutes). This concentration permits adjustment of the carbon potential of the atmosphere to a value equal to the carbon concentration in the austenite at saturation corresponding to the chosen enrichment temperature.

[0020] at the end of the enrichment phase, the atmosphere of the furnace is replaced by nitrogen. The carbon flux transferred to the interface is then zero. The temperature and the duration of this diffusion phase into the steel at a zero transfer flux are chosen to obtain the desired surface carbon concentration profile.

[0021] This diffusion phase ends at the temperature chosen for the quenching of the pieces.

[0022] The introduction of ammonia immediately before the beginning of carbon enrichment permits the creation of a perfectly deoxidized surface by the hydrogen atoms resulting from the decomposition of the ammonia molecule at the steel surface:

NH3=>N°+3H°

[0023] This transient reaction is followed by the formation of molecular nitrogen and hydrogen, resulting in a mixture containing 75% hydrogen and 25% nitrogen.

2N°=>N2

2H°=>H2.

[0024] This mixture is highly reducing at these temperatures. It permits avoidance of the formation of oxides during the short period of carbon potential increase at the time of introduction of the CO—H₂ mixture used for enrichment.

[0025] The absence of adsorbed oxygen and oxidized layer permits suppression of interfacial resistance to carbon transfer. This absence of oxygen, in combination with efficient circulation of the gaseous mixture at the surface of the pieces (agitation of the gas inside the furnace is assured), permits the surface carbon concentration to reach the concentration corresponding to the enrichment carbon potential in less than three minutes after it was reached by the atmosphere.

[0026] Another advantage that is achieved with this maximum rate of enrichment and by the absence of oxidation is that the treatment of the invention permits control of material transfer in a rigorous manner, and thus monitor and simulate the concentration profiles obtained in real time. As a matter of fact, each phase of the treatment corresponds to rigorous limiting conditions:

[0027] during the heating, the carbon flux is zero,

[0028] during the enrichment phase, the transfer of carbon is at constant surface concentration equal to the saturation concentration of austenite,

[0029] during the diffusion phase, the carbon flux is zero.

[0030] In summary, the method of the invention has the following advantages:

[0031] carbon is transferred at the maximum rate,

[0032] no surface oxidation is observed,

[0033] it permits rigorous control of the carbon transferred by control and regulation of the carbon potential of the atmosphere,

[0034] it permits simulation and control of the carbon concentration profiles at each instant due to the control of the limiting conditions at the interface for each treatment phase. Analyses show that there is no nitrogen introduced into the steel, the concentrations measured over the first 100 microns are less than 0.02%, which conforms to thermodynamic predictions.

[0035] According to special modes of embodiment, simultaneously with the ammonia, other products are introduced which reinforce the reducing nature of the atmosphere, such as hydrocarbons, for example, methane. Such an addition permits pre-enrichment of the carbon surface before the introduction of the CO—H₂ mixture. Finally, any source of atomic hydrogen in the atmosphere before carbon enrichment leads to the described phenomena.

[0036] According to another characteristic of the invention, in an application to the carbonitridation of steels, ammonia is introduced at a controlled flow rate at a temperature below 880° C. during the diffusion phase when the carbon flux is zero, so as to create a nitrogen concentration gradient. This operation is carried out according to the usual carbonitriding practices, according to the following method of operation:

[0037] heating the pieces up to 920° C. in nitrogen, followed by the introduction of ammonia,

[0038] carbon enrichment between 930 and 980° C.,

[0039] carbon diffusion in a nitrogen atmosphere,

[0040] decrease of the temperature to 880° C.,

[0041] maintenance of the temperature at 880° C. with the introduction of ammonia,

[0042] quenching of the steel. 

1. Method of carbon enrichment of steels with carbon monoxide, comprising a heating step in a gas-phase furnace up to the enrichment temperature, followed by an enrichment step by the introduction of a gas containing carbon monoxide, wherein atomic hydrogen is formed in the said gaseous phase before the enrichment step, in a quantity sufficient to suppress the surface resistance to the transfer of carbon and to avoid surface oxidation of the steel.
 2. Method according to claim 1, wherein chemical compounds are introduced into the said gaseous phase, the thermal decomposition of which leads to the formation of atomic hydrogen, at least transitorily.
 3. Method according to claim 2, wherein ammonia is introduced.
 4. Method according to claim 3, wherein a hydrocarbon is introduced simultaneously with the ammonia, for example, methane.
 5. Method according to claim 4, comprising a carbonitridation step, wherein ammonia is introduced at a controlled flow rate at a temperature below 880° C. during the diffusion phase when the carbon flux is zero, so as to create a nitrogen concentration gradient.
 6. Method according to claim 1, wherein atomic hydrogen is formed by physical means in the form of cold plasmas or hot plasmas, in discharges or post-discharges.
 7. Method according to claim 1, wherein the carbon concentration profiles are monitored in real time.
 8. Method according to claim 1, wherein the carbon concentration profiles are simulated in real time. 